A mesoscale strength model for silica-filled polydimethylsiloxane based onatomistic forces obtained from molecular dynamics simulations

We present a novel mesoscale model that describes the tensile stress of sil ica-filled polydimethylsiloxane (PDMS) under elongation. The model is based on atomistic simulations of a single chain of PDMS, interacting with itsel f and/or a hydroxylated silica surface. These simulations provide estimates of the microscopic forces required to stretch or uncoil a chain of PDMS, o r detach it from a silica surface. For both stretching and detachment, we f ind that the internal potential energy is linear with the distance the chai n end is moved, albeit with differing slopes. From these calculations and r ecent atomic force microscopy (AFM) experiments, we conclude that the force s are constant. We apply this analysis to the case of uncrosslinked, silica -filled PDMS systems and develop a mesoscale, inter-particle strength model . The strength model includes the atomistic forces determined from the simu lations, a small entropic component, and a Gaussian probability distributio n to describe the distribution of chain lengths of PDMS strands connecting two silica particles and the chain lengths in the free ends. We obtain an a nalytic stress/strain expression whose predictions agree with experiment. T his model also suggests mechanisms to explain the phenomena of hysteresis a nd permanent set. (C) 2000 American Institute of Physics. [S0021-9606(00)52 341-1].